Last week a storm broke the dyke around the plant, sparking fears the paraxylene (PX) it makes could spill.

PX is used in fabric manufacture and can be highly toxic.

About 12,000 residents took part in the protest, some of them moving across the city chanting slogans and waving banners.

Xinhua said the city's top official, Tang Jun, had tried to calm the crowd on Sunday but the protesters showed no sign of dispersing.

There were no reports of injuries in the scuffles during which riot police were deployed to shield the municipal government office.

Calls for protesters to gather on Sunday for a "group stroll", as the rally was termed, had reportedly been circulating on social networks.

"Group strolls" have become a favoured tactic for Chinese people to show discontent with the government.

'PX out!'

Photographs posted on the internet on Sunday showed protesters, including children, marching under such banners as "I love Dalian and reject poison" and "Give me back my home and garden! PX out! Protect Dalian!".

One picture showed three men standing on top of a police van in front of People's Square and a person in a skeleton costume surrounded by hundreds of men and policemen, Reuters news agency reports.

On Monday, residents living near the PX plant had to be evacuated after storm waves breached a dyke protecting it.

The dyke was repaired but concern rose among local people, and reports suggested the plant may have been operating illegally months before it received mandatory environmental approval.

PX is used to make plastics, polyester and cleaning products, and can damage vital organs after long-term exposure.

A Dalian resident, who declined to be named, told Reuters news agency: "We know that the typhoon caused some leak of poisonous chemicals from the PX project and we are all worrying about it because it is a threat to our life."

Local people hoped their protest would "push the government to do something as soon as possible to dispel" the concern, the resident added.

Weibo, China's version of Twitter, was being censored by the authorities to block searches for the terms "PX", "Dalian" and "Dalian protests".